1. Field of the Invention
This invention relates to silicate- and magnesium-free stabilizer mixtures for stabilizing aqueous peroxide bleaching baths, and to a process for bleaching cellulose fibers, if desired in combination with synthetic fibers, with aqueous hydrogen peroxide solutions stabilized with silicate- and magnesium-free stabilizer mixtures.
Cellulose fibers and mixtures consisting of cellulose and synthetic fibers are frequently bleached with hydrogen peroxide in alkaline medium. However, alkaline peroxide baths are not stable. The rate at which they decompose increases with increasing temperature, so that stabilizers have to be added to enable the peroxide to be utilized with virtually no losses for the bleaching process. Stabilizers based on waterglass, i.e. silicate, in combination with magnesium ions are normally used for caustic-alkaline peroxide bleaching.
Waterglass-containing stabilizers have an outstanding anticatalytic effect against heavy metals, such as iron, copper, nickel, manganess and compounds thereof. Heavy metals and their compounds are able to enter and contaminate the bleaching bath through the water, the chemicals, the apparatus or even the material being bleached and to cause catalytic decomposition of the peroxide which in turn results in fiber damage. Further advantages of stabilizer mixtures containing waterglass include outstanding bleaching effects and the absence of husks in the cellulose fiber components. However, a serious disadvantage of silicate-containing stabilizers is that calcium and/or magnesium silicates are precipitated where hard water is used, resulting in a hard feel of, and in the formation of marks, on the material and also in difficulties during dyeing and printing. In addition, unwanted silicate deposits can build up in machine units. In the bleaching of starch-sized material, which has not been enzymatically desized, inadequate desizing effects are obtained despite the use of sodium persulfate.
2. Discussion of Related Art
The result of these disadvantages of silicate-containing stabilizers is that many purely organic silicate-free stabilizers have been developed. Thus, U.S. Pat. No. 3,860,391, for example, describes organic stabilizer mixtures consisting of amino-lower alkane phosphonic acids, polyhydroxy or hydroxycarboxylic acids and polyaminocarboxylic acids. By comparison with silicate-containing stabilizers, however, the desired effects in regard to desizing, whiteness and the final average degree of polymerization are still not obtained with silicate-free mixtures. Accordingly, an object of the present invention is to provide silicate- and magnesium-free stabilizer mixtures which do not have any of the described disadvantages of silicate-free stabilizer mixtures in regard to desizing, whiteness and resulting average degree of polymerization.